Problem: 10 notebooks cost $9.30. Which equation would help determine the cost of 2 notebooks?
There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 10 notebooks. We want to know the cost of 2 notebooks. We can write the numbers of notebooks as a proportion: $\dfrac{10}{2}$ We know 10 notebooks costs $9.30. We can let $x$ represent the unknown cost of 2 notebooks. The proportion of these costs can be expressed as: $\dfrac{\$9.30}{x}$ The cost changes along with the number of notebooks purchased, and so the two proportions are equivalent. $\dfrac{10}{2} = \dfrac{\$9.30}{x}$